Thin polished plates in the form of silicon wafers are a very important part of modern technology. The requirements for flatness and thickness uniformity of these wafers are becoming more and more stringent as the printed device feature sizes are shrinking. Therefore, the metrology of these parameters is very important for development and manufacturing. Other examples for opaque polished plates are magnetic disc substrates, gauge blocks, and the like. While the technique described here refers mainly to wafers, it is to be understood that the technique also is applicable to other types of test pieces with comparable characteristics.
There exist a variety of techniques to address the measurement of shape and thickness variation of wafers. Currently, the technique most commonly used is based on capacitive distance sensors (ASTM standards F 1530-94 and F 1390-97). For measurement, two sensors are placed near either side of the wafer and scanned together across the wafer surface, thus acquiring a distance map from each wafer surface to the corresponding capacitive sensor. From these two distance maps, the thickness variation and shape are calculated. For capacitive sensors, the achievable height accuracy and spatial resolution are limited and are no longer adequate for future wafer requirements.
A different point-sensor technique uses optical triangulation sensors for the distance measurements. While the spatial resolution is better, the height accuracy is not sufficient. In addition, the scanning time per wafer is very long for high spatial resolution.
A further technique is based on infrared interferometry using light of a wavelength where the wafer is transparent as disclosed in the U.S. Pat. No. 5,909,282 to Kulawiec, and in international patent application No. WO97/45698. One of the two interfering beams is passed through the wafer twice more than the other beam, thus producing an interferogram with a phase distribution proportional to the optical thickness variation of the wafer, which is the product of the refractive index of the wafer material and the geometric thickness. The shortcomings of this technique are that highly doped wafers are not transparent, even in the infrared and cannot be measured, and that only the thickness variation, but not the shape, is obtained.
In another technique (disclosed in the U.S. Pat. No. 5,502,564 ) based on multi-spectral interferometry, the wafer is placed close to a plane reference surface and is illuminated by broadband light at oblique incidence. This is done either from one side only, and only the front surface map for a chucked back surface can be determined, or from both sides, where both surface maps are measured and thickness variation and shape is obtained. The broadband nature of the light allows for obtaining the distance between the wafer surface and the reference surface by analyzing the spectral modulation of the reflected light. This technique suffers from the need to place the wafer close to the reference surface, which leads to significant difficulties in wafer handling. Furthermore, using large angles of incidence of the illumination leads in effect to a desensitizing of the measurement, thus reducing measurement precision. In addition, it is difficult to combine the two single-sided surface maps to calculate the thickness variation and shape with the necessary overlay accuracy.
Another interferometric technique using oblique incidence is described in the U.S. Pat. No. 4,653,922 to Jarisch. In this patent, the interferometric test beam is reflected at one wafer surface, and then after reflection, is directed to the second wafer surface, such that the interferogram shows the sum of both wafer surface height distributions, which is related to the thickness variation of the wafer. The drawbacks here are the requirement for optical components much larger than the wafer, a desensitizing due to the large angle of incidence, the long air path, and the lack of shape information.
Further interferometric techniques at grazing incidence are described in the German patent application disclosure to No. DE 196 02 445 A1, in U.S. Pat. No. 6,249,351 to deGroot, and in international patent application Nos. WO 00/79245 A1 and WO 01/77612 A1. In these, the wafer is illuminated from both sides at grazing incidence, where the beam splitting element and beam recombining element are diffraction gratings. Limiting the measurement precision in these systems are the desensitizing of the measurement due to the grazing incidence, the effect of air turbulence in the large non-common air path between the test and reference beams, and the difficulty to properly combine the two single-sided measurements for the thickness and shape calculations.
A double-sided interferometric technique at normal incidence is described in the U.S. Pat. No. 6,504,615 to Abe. Two Fizeau interferometers are employed to measure the shape of both wafer surfaces simultaneously, where the wafer is placed upright between the two reference surfaces. In addition to the two single-sided interferometric surface maps, the wafer thickness is measured at a set of several points, e.g. with capacitive sensors. The individual surface maps derived from the interferograms are then combined with the thickness data to obtain full wafer thickness maps. The added thickness measurements are necessary to obtain the wedge or linear thickness variation component of the wafer, which is uncertain from the interferometric measurements only, since its measurement is affected by the tilt between the two reference surfaces. This tilt is very sensitive to mechanical instability in the sub-micron level, and cannot be assumed to be sufficiently stable. The shortcomings of this technique are the need for the additional capacitive thickness measurements, and the difficulty to combine the front and back surface maps with the necessary accuracy. Furthermore, any residual shape errors of the reference surfaces, such as a sag or high frequency waviness, affects the wafer measurements and reduces their accuracy.
Another double-sided interferometer at normal incidence for the testing of magnetic disk substrates is described by K. Levotsky in INTERFEROMETER MEASURES BOTH SIDES OF DISK, Laser Focus World, September 1997, P. 52-53. There, the illumination is switched sequentially between the two sides of a wafer, and only one camera is used. Thus, simultaneous acquisition is not possible, which may lead to measurement errors due to a drift of the thin disk between the measurements.
The U.S. Pat. No. 6,061,133 to Freischlad discloses a low coherent noise interferometer system employing a light source useful in interferometer systems to provide improved performance.
It is desirable to provide an improved method and apparatus for rapidly measuring the thickness variation and shape of wafers, or more generally, polished opaque plates, at high accuracy levels and spatial resolution, without the aforementioned shortcomings.